Mercury contact relay



Jui? 1' 1.941- H. c. HARRISON E-rAL 2,247,493

MERCURY CONTACT RELAY Filed Nov. 2, 1939 Patented July 1, 1941.

MERCURY CONTACT RELAY 1 Henry C. Harrison, Port Washington, N. Y., andCharles E. Pollard, Hohokus, N. J., assgnors to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication November 2, 1939, Serial No. 302,526

'(Cl. 20G- 112) 11 Claims.

This invention relates to electromagnetic devices and more particularlyto relays in which circuits between the contact terminals thereof areestablished by means of mercury.

Switches in which circuits are established bcv tween contact terminalsthrough the m'edium of mercury possess many advantages over switchesofthe contact spring type in that the tendency of arcing of the contactsand the consequent Contact deterioration is reduced; the tendency ofContact chatter is much reduced or practically eliminated; the currentcarrying capacity is materially increased without requiring a greatincrease in the size oi' the switch; a smaller operating current isrequired; the switch is readily adaptable for delayed circuit closing oropening or for providing many contact combinations required for circuitcontrol such as transfer contacts, continuity contacts, etc., and aplurality of such switches having different time constants may be easilyassembled for operation by the same electromagnetic means.

It is therefore the objectJ of the present invention to provideelectromagnetically operated switches of the mercury contactl type inwhich the above speciiied and other advantages are readily andeconomically attained.

For attaining this object and in accordance With one embodiment of thepresent invention a small envelope of glassor other suitable material isprovided in which a light plunger armature of magnetic material isslidably positioned and which supports a globule of mercury. Sealedirrtoone end of the envelope are two or more inwardly extending contactterminals or electrodes which may be bridged by the globule of mercury..Preferably to reduce the deterioration of the contacts and thecontamination of the mercury, the envelope is evacuated and relled withan inert gas, such as hydrogen, neon, argon, etc. at a suitablepressure. For securing diiierent types of contact closures, suchA as forexample, quick make-quick break, quick make-slow break, and slowmake-quick break and diierent types of contact openings, such as quickbreak-quick make, slow break-quick make and quick .breakslow make,ceramic insulators of diierent configur-ations may be supported by theelectrodes in one end of the envelopes or the depending ends disposedpole-pieces of an electromagnetic circuit,l although it will be apparentthat other types of magnetic circuits, such as a solenoid coilsurrounding the envelope might; be employed. It will also be apparentthat a plurality of switches having different operating characteristics.could have their plunger armatures Iactuable by a common magneticcircuit by providing a plurality of aligned openings in the poleypiecesof the magnetic circuit in which such switches are positioned.

As an alternative construction the envelope may contain a pool ofmercury in the bottom thereof in which one of the depending electrodesis immersed and the other electrode may be terminated in a coil ofmagnetic material which when subjected to a magnetic iield eithercontracts or expands to break or make contact With the pool of mercury.

For a more complete understanding of the invention, reference may be hadto the following detailed description taken in connection with theaccompanying drawing in which:

Fig. 1 shows a relay having a plurality of mercury type switch elements;

Fig. 2 is a partial cross-sectional View of one of the switch elementsarranged for a quick make-quick break of its normally open contacts;

Fig. 3 is a side elevational view and Fig. 4 is a bottom end view of theceramic insulator of the switch disclosed in Fig. 2;

Fig. 5 is a cross-sectional view of a modified type of ceramicinsulator;

Fig. 6 is a partial cross-sectional View of a switch element arrangedfor a quick make-slow break of its normally open contacts;

Fig. 7 is a partial cross-sectional view of a switch element arrangedfor a slow make-quick break of its normally open contacts;

Fig. 8 is a partial cross-sectional View of a switch element arrangedfor a quick breakquick make of its normally closed contacts;

Fig. 9 is a partial cross-sectional` view of a switch element arrangedfor a slow break-quick make of its normally closed contacts;

Fig, 10 is a partial cross-sectional view of a switchelement arrangedfor a quick break-slow make of its normally closed contacts;

Figs. 11 to 14, inclusive, show modied forms of lswitch elementsarranged for quick recycling of which the element of Fig. 11 is arrangedto close its contacts slowly and to open them quickly; the element ofFig. 12 is arranged to close certain of-its contacts quickly, tovcloseother contacts slowly and to open all of its contacts quickly; theelement of Fig. 13 is arranged to open its normally closed contactsquickly and to close Ithem slowly; and the element of Fig. 14 isarranged to open all of its normally closed contacts quickly and toclose certain of them quickly and others of them slowly;

Fig. 15 is a cross-sectional view of the envelope used in switchelements of the types disclosed in Figs. 6, 'T and 9 to 14, inclusive;

Fig. 16 shows a modified type of switch element in which a conductivepath through a pool of mercury is established by the elongation of acoil of magnetic-spring material under the influence of a magneticcircuit; and

Fig. 17 shows the switch element of the same type as disclosed in Fig.16 in which a normally established path through a pool of mercury isdisrupted by the construction of a coil of magnetic spring material.

A relay employing switch elements of the mercury contact type isdisclosed in Fig. 1. This relay comprises a core I to the forward end ofwhich is suitably secured a pole-piece plate 2 and to the rear end ofwhich is secured a forwardly extending return pole-piece 3. The returnpole-piece is offset in its middle portion to provide clearance for therelay winding 4 surrounding the core I, is bent at right angles at itsrear end to form a mounting bracket portion 5 by which the relay may besecured to a relay rack mounting plate and has its forward end 6positioned parallel to and suitably spaced from the pole-piece plate 2.Secured by screws I and in sulatedly supported upon the rear end of coreI are terminal lugs 8 and 9 to which the terminals of the coil 4 areconnected. Other terminal lugs may be provided, if desired, forconnection with the electrodes of the switch elements supported by thepole-pieces as hereinafter described. `As illustrated the pole-pieces 2and 6 are provided with aligned openings in which four mercury contactswitch elements are positioned. It will, of course, be obvious thatprovision may be made for a single switch element or any desired numberof such elements.

The switch elements may be of any of the types disclosed in Figs. 2, 6to 14, inclusive, 16 and 17. The switch elements disclosed in Figs. 2and 6 to 10 each comprises an envelope I0 made of glass or any othersuitable material which will not amalgamate with the mercury enclosedtherein, two electrodes II and I2 sealed through one end thereofandextending into the inside of the envelope, a plunger armature I3 ofmagnetic material supported upon a spring I4 positioned in the bottom ofthe envelope, the plunger being of slightly smaller diameter than theinside diameter of the envelope whereby it is free to slide within theenvelope, a globule of mercury I5 positioned adjacent to one face of theplunger and an insulating cylinder I6 preferably of porous unglazedceramic material not easilycontaminatedby the mercury supported uponelectrodes Il and I2 in one end of the envelope.

The insulating cylinder I6 for the switch element of Fig. 2, disclosedin detail in Figs. 3 and 4,

is provided with two holes extending longitudinally thereof throughwhich the switch electrodes may extend, the electrodes fitting tightlyenough in such holes to support the cylinder securely in the desiredposition in the upper end of the envelope. The lower end of the cylinderis cut away as indicated to provide a half cylindrical recess II for apurpose to be later described.

As disclosed in Fig. 2, the plunger I3 is nor` mally so positionedwithin the envelope that when the switch element is mounted in thealigned openings of the pole-pieces 2 and 6 of the electromagnet, itsupper surface will be substantially in the plane of the upper face ofthe pole-piece 6 in which position the globule of mercury I5 is engagedwith the end of the electrode I2. When, however, a magnetic potential isestablished between the pole-pieces 2 and 6, a flux path will beestablished through the plunger I3 and the plunger will be drawnupwardly in the envelope in an attempt to center itself between thepolepieces 2 and 6. This movement of the plunger moves the globule ofmercuryA upwardly, a tongue of the mercury being forced into the recessI`I of the insulating cylinder IB until it engages the end of electrodeII which extends downwardly into the recess. Since the recess is shallowand the electrode II extends nearly to the lower end thereof, themovement ol` the mercury globule effects quick closure between theelectrodes II and I2. Reversely when the pole-pieces 2 and 5 becomedemagnetized, the plunger I3 is freed to drop upon the upper end ofspring I4 which in this case serves as a shock absorber and the tongueof mercury will be withdrawn quickly from the recess Il due to thespring action of the surface tension of the mercury, thus forcing theplunger downwardly and effecting a quick opening between the `electrodesII and I2. This switch element thus affords a quick make-quick break ofnormally open contacts.

The switch element disclosed in Fig. 8 is substantially similar to thatAdisclosed in Fig. 2 except that the insulator I6 is shorter and springI4 is expanded to normally hold the plunger I3 in the positiondisclosed, whereby the globule of mercury I5 is normally in contact withthe ends of both electrodes II and I2 thus effecting a normal circuitclosure. When, however, a magnetic potential is created between thepole-pieces 2 and 6, the plunger I3 will be drawn downwardly in theenvelope to compress the spring I4 and due to its surface tension thetongue of mercury will withdraw quickly from the recess I'I thus quicklyopening the contact between the mercury and the electrode II. Reverselywhen the polepieces become demagnetized, the plunger is quickly forcedupwardly by the spring I4 to force a tongue of mercury into the recessI1 to again effect a circuit closure. This switch element thus 'affordsa. quick break-quick make of normally closed contacts.

The switch element of Fig. 6 is substantially the same as that disclosedin Fig. 2 with the exception of the arrangement of electrodes II and I2with respect to the recess in the insulator I6. In this case therecessed portion I1 of the insulator is made much deeper and thedepending Vend of electrode II extends substantially to the lower end ofthe recessed portion II. When the plunger I3 is electromagneticallyactuated toward the upper pole-piece 2, a tongue of mercury from theglobule I5 is forced into the recess I1 and quickly engages the end ofelectrode II, thus effecting a quick contact closure. As the plungermoves upwardly the tongue of mercury becomes extended until itcompletely fills the recess. Reversely when the pole-pieces aredemagnetized and the plunger I3 drops down upon the buffer spring I4 thesurface tension of the mercury globule will tend to withdraw the tongueof mercury from the recess l1. To delay this action the envelope isprovided with a capillary groove I3, shown most clearly in Fig. 15, inthe wall of the envelope opposite the recessed portion Il of insulatorI6 through which the .gas flows slowly the depending portion ofelectrode II protrudesv for only a short distance below the top of therecessed portion I1 of the insulator I6. When the plunger I3 iselectromagnetically actuated towards the pole-piece 2, the tongue ofmercury from the globule I5 is forced into recess I1 but due to the slowflow of gas through the capillary groove I8, past the mercury globule,the time required for the tongue to'completely fill the recess andengage the end of electrode II is extended, thus effecting a slowclosure between the electrodes II and I2. Reversely, when the polepiecesare demagnetized and the plunger I3 is freed to drop down upon thebuffer spring Il, the end of the tongue of mercury will, due to thesurface tension of the mercury withdrawn from the end of electrode II,quickly thus forcing the plunger downwardly and effecting a quick breakbetween the electrodes. This switch element thus affords a slowmake-quick break of normally open contacts.

The switch element of Fig. 9 is substantially the same as that disclosedin Fig.8 except that the capillary groove I8 is provided, the recess I'Iin the insulator I6 is made deep and the dependpole-pieces arethereafter demagnetized, the plunger I3 will at first be restoredquickly by the spring I4 and will tend to force a tongue of mercury intothe recess I1. 'The further restoring movement of the plunger will thenbe delayed by the slow extension of the tongue of mercury into therecess as determined by the ilow of gas through the groove I8, untilfinally the end of the tongue engages the end of! electrode II toreestablish the contact between the electrodes II and I2 through themercury. This switch element thus affords a quick break-slow make ofnormally closed contacts.

, As an alternative the semicylindrical recess I1 of the insulator maybe omitted as disclosedln Fig. 5 and the electrode II may be extendeddifferent distances through the hole in the insulator IB dependent uponthe circuit closing or opening characteristic desired, the remainingportion of vthe hole I1' below the lower end of electrode II serving asa recess into or from which the tongue of mercury is forced orwithdrawn.

While the switch elements shown in Figs. 2 to 10, inclusive, have beenillustrated aspositioned ing end of electrode II extends throughout theentire length of the recess. Normally the plunger `I3 is held in theposition shown by the spring I4 s o that a tongue of .the mercuryglobule I5 is forced up into and completely lls the recess,

thus electrically connecting the electrodes Il and f I2. When theplunger I3 is electromagnetically drawn against the tension of spring Iltoward the pole-piece 6, the tongue of mercury will tend 'A to withdrawfrom the recess but will be retarded in its withdrawal by the slow flowof gas through the grooveg I8 and since the tongue of mercury willengage the end `of electrode II until it is entirely withdrawn from therecess, a slow break between the 'electrodes II and I2 is effected. Whenthe pole-pieces become demagnetized, however, the plunger I3 will firstbe restored rapidly by the spring I4 thereby quickly engaging themercury globule I5 with the ends of electrodes II and I2 and willthereafter move more slowly until a tongue of mercury again lls therecess I1. This switch element thus affords a slow breakquick make ofnormally closed'contacts.

The switch element of Fig. 10 is also substanftially similar to that ofFig. 9 with the exception 'that the depending endof electrode II extendsbut a short distance into the recess I1 of the insulator I6. In thenormal position of `the plunger I3 as disclosed, a tongue of the mercuryglobule is forced into and completely fills the jrecess, thuspconnectigthe ends of the electrodes. When theeplunger I3 is electromagneticallydrawn downwardlytoward the pole-piece 6, the tongue' of mercury willtend to withdraw from the recess.

but will be retarded in its withdrawal by the slow `flow of gas throughthe groove IB but vsince the.

tirely.` withdraw from the recess. Wheri the vertically with theirplungers in the lower portions thereof, these elements will function ifin. verted or positioned horizontally.

Figs. 11 to 14, inclusive, disclose switch elements which are notprovided with insulating cylinders in the upper portion of the envelopeswhereby more space is provided in the upper portion of the envelopes toenable a luickerrecycling through the more rapid collapse of theglobules of mercury. Figs. 11 and 12 disclose switch elements havingnormally open contacts and Figs. 13 and 14 disclose switch elementshaving normally closed contacts.

In accordance withFig. 11 upon the magnetization of the pole-pieces 2and i, the plunger I3 is drawn upwardly toward the pole-piece 2, thusraising the'globule of mercury I5 into engagement with the ends ofelectrodes II and I2. 'Ihe movement of the globule is, however, delayeddue Yto the slowv passage of gas through the capillary Amake-quick breakof normally open contacts.

In accordance with Fig. 12, threeelectrodes I II, I2 and I9 areprovided, electrodes II and I2 terminating just above the upper surfaceof the mercury globule I5 and the electrode Il terminating at a greaterdistance above the surface of the globule. When the pole-pieces 2 and 6become magnetized, the plunger I3 is drawn up` wardly toward thepole-piece 2 thus raising the globule of mercury into engagement withthe, ends of electrodes/ II and I2 to effect a quick closure betweenthem. 'I'he further movement of the globule is, however, delayed due tothe slow passage of gas through the capillary groove I! past the globulewhereby a slow closure between the electrodes II' or I2 and electrode I9is, effected. When the pole-pieces are demagnetized, the plunger dropsdown upon the buffer spring I4 and the globule collapses quickly todisengage all of the electrodes. This switch element therefore' affordsboth a quick and slowl make and' a quick break of its normally opencontacts.

The switch element of Fig. 13 is so constructed that plunger 13 is'normally held in the position disclosed by the spring Il in\whlchposition the globule of mercury I makes contact with the ends ofelectrodes II and I2. When the polepieces 2 and 6 become magnetized, theplunger is drawn downwardly toward the pole-piece E thereby compressingthe spring I4 and quickly withdrawing the mercury from engagement withthe electrodes. Upon the later demagnetization of the pole-pieces thespring I4 tends to restorethe plunger to its normal position, but isdelayed in doing so by the slow leak of gas past the globule through thecapillary groove I8. A delay interval is thus introduced before theplunger rises sufliciently to move the globule of mercury intoengagement with the electrodes. This switch element therefore affords aquick break and a slow make of itsnormally closed contacts.

The switch element of Fig. 14 is similar to thatl of Fig. 13 except thatthree electrodes II, I2 and I9 are provided, the ends of electrodes IIand I2 being normally deeply immersed in the mercury globule I5 and theend of electrode I9 being less deeply immersed. In the operation of thisswitch element when the pole-pieces 2 and 6 become magnetized and theplunger I3 is drawn toward the pole-piece 6, against the tension ofspring I4, the mercury globule I5 follows it to quickly disengage all ofthe electrodes. When thereafter the pole-pieces become demagnetized, thespring I4 tends to restore thplunger to its normal position but isdelayed in doing so by the slow leak of gas past the globule through thecapillary groove I8. Due, however, to the fact that electrodes II and I2 are long and in the operated position of `the plunger their ends areclose to the upper surface of the globule, the first part of therestoring movement of the plunger will quickly bring the globule intoengagement with the ends of such electrodes, but electrode I9 being muchshorter will not be engaged by the globule until near the end of therestoring movement of the plunger. This switch element thus provides aquick break of all of its normally closed ucontacts, a quick makebetween certain of its contacts and a slow make between other of itscontacts. y

In Figs. 16 and 17 a further embodiment of thev invention has beendisclosed in each of which the globule of mercury I5 is deposited in thebottom of envelope I0 and one of the electrodes 2l) has its end immersedin the globular, The other electrode 2l is made of magnetic springmaterial or has.l that portion thereof depending Within the envelopemade of such material and provided with a helix 22.

The switch element disclosed in Fig. 16 is of the normally open circuittype. The electrode 2l thereof extends downwardly to a point above thelower pole-piece S and is then coiled upwardly about the downwardlyextending portion to form the closely coiled helix portion 22, with theend of the electrode extending from the top coil of the helix downwardlythrough the helix and terminating just above the upper surface ofthe'mercury globule. When the pole-pieces 2 and 6 become magnetized, thelower portion of the helix is drawn toward the lower pole-piece 6thereby moving the lower end of the electrode 2| into engagement withthe mercury to quickly establish a circuit through the mercury betweenthe electrodes 2U and 2I. When the pole-pieces become demagnetized, thehelix contracts quickly and withdraws the end of electrode 2l from themercury thus opening the connection between the electrodes. v

The switch element of Fig. 17 is of the normally closed circuit type andtherefore the end of the electrode 2I is normally immersed in themercury globule I5. The helix 22 there of has its coils loosely wound.When therefore the polepieces 2 and 6 become magnetized, the adjacentcoils of the helix 22 become attracted toward each other therebycontracting the helix to such an extent as to disengage the end ofelectrode 2I from the mercury to effect a circuit opening. Thereafter,upon the demagnetization of the pole-pieces the helix expands to againengage the end of electrode 2I with the mercury.

While the invention has been illustrated by certain specific embodimentsthereof, other ar-4 rangements of electrodes and magnetically actuatedmembers and the association of the globules of mercury therewith forsecuring other types of circuit control will be apparent to thoseskilled in the art. It is therefore to be understood that the scope ofthe invention is limited only by the bounds set forth in the appendedclaims.

What is claimed is:

1. In an electromagnetic structure having an electromagnetic field, acontact element comprising a sealed envelope, terminals extending intosaid envelope, a plunger of magneticmaterial responsive to saidelectromagnetic field and freely movable in said envelope and a globuleof mercury supported on the end of said plunger in engagement with theinner wall of said envelope and movable thereby to control the bridgingof said terminals.

2. In anelectromagnetic structure having an electroma gnetic field, acontact element comprising a sealed envelope, terminals extending intosaid envelope, a plunger of magnetic material responsive to saidelectromagnetic field and freely movable in said envelope and a globuleof mercury supported on the end of said plunger in'engagement with theinner wall of said envelope and movable thereby into bridging engagementwith said terminals.

3t In an electromagnetic structure having an electromagnetic field, acontact element comprising a sealed envelope, terminals extending intosaid envelope, a plunger of magnetic material responsive to saidelectromagnetic field and freely movable in said envelope and a globuleof mercury supported on the end of plunger in engagement with the innerWall of said envelope and movable thereby out of normal bridgingengagement with said terminals.

4.' In an electromagnetic structure having an electromagnetic field, acontact element comprising a sealed envelope,` an insulator supported inone end of said envelope, the inner end of which is provided witharecessed portion, a pair of terminals of different lengths extendingthrough the end wall of said envelope and through said insulator, theinner end of the longer terminal being exposed beyond' the inner end ofsaid insulator and theinner end of the shorter terminal being exposed inthe recessed portion of said insulator, a plunger of magnetic materialresponminals of different lengths extending through insulator, the innerend of the longer terminal being exposed beyond the inner end of saidinsulator and the inner end of the s orter terminal being exposed insaid recessed portion of said insulator, a-plunger of magnetic' materialresponsive to said magnetic field and freely movable in said envelopeand a globule of mercury associated with said plunger normally inengagement with said longer terminal and movable 'by said plunger intothe recessed portion of said insulator into engagement with said shorterterminal. 1 f

6. vIn an electromagnetic structure vhaving an electromagnetic field, acontact element comprising a sealed envelope, an insulator supported inone end offsaid envelope, the inner end of which isprovided with arecessed portion, a pair of terminals of different lengths extendingthrough the end wall of said envelope and through said insulator, theinner end of the longer terminal being exposed beyond the inner end ofsaid insulator and the inner'end of the shorter terminal being exposedin said recessed portion of said insulator, a plunger of magneticmaterial responsive to said magnetic field and freely movable in saidenvelope and a globule of mercury associated with` said plunger normallyin engagement with the inner end of said insulator and filling therecessed portion thereof to bridge sjaid termina1s, saiduglobule beingmovable by the "attr-acted movement of said plunger out of engagementWith said Iterminals.

7. In an electromagnetic structure having an electromagnetic field, acontact element comprisingl a sealed envelope, an insulator supported inone end of said Yenvelope the inner'end of which is provided with adeeply recessed portion, a pair of terminals vofdiiferent lengthsextending through the end Wall of said envelope and through said.insulator, the inner end of the longer terminal. being exposed beyondthe inner end of said insulator and"`the inner end of the shorterterminal exten `ng substantially throughout the length of saidl recessedportion of said insulator, a plunger of magnetic material responsive tosaid magnetic -el'l and'freely Vmovable lin said envelope, a globule ofmercury prising a sealed envelope, an insulator supported in one end ofsaid envelope, the inner end of which is provided witha deeplyrecessedportion, a pair of terminals of different lengths extending through theend wall of said envelope and through said insulator, the inner end ofthe longer terminal being exposed beyond the inner end of saidinsulator-and the inner end of the 'shorter terminal being exposed,onlyjust beyond the bottom of the recessed portion of said insulator, 4aplunger of magnetic material respon- G sive to' said magnetic field andfreely movable in said envelope, a globule of mercury associated withsaid plunger and movable thereby to control the bridging of saidterminals and a capillary groove in the inside wall of said envelope forcontrolling the rate of movement of said globule of mercury. f

9. In an electromagnetic structure having an electromagnetic iield, a.contact element comprising a sealed envelope, an insulator supported inone end of said envelope, the inner end of which is provided with adeeply recessed portion, a

pair of terminals of diierent lengths extending through the end wall ofsaid envelope and through said insulator, the inner end of the longerterminal being exposed beyond the inner end of said insulator and theinner end of the shorter terminal extending substantially the lengthofvsaid recessed portion of said insulator, ,a plunger of magneticmaterial responsive to said magnetic field and freely `movable,in saidenvelope, a globule of mercury associated with said plunger and n mallyin engagement with said longer terminal and movable by the attractedmovement of said plunger 'into the recessed portion of said insulatorinto engagement with said shorter terminal, and a capillary groove inthe inside wall of said envelope for delaying the flow of said mercuryglouble from the recessed porition of said insulator upon the retractivemove,- ment of said plunger.

10. In an electromagnetic structure having an electromagnetic field, acontact element comprising a sealed envelope, an insulator supported inone end of said envelope, the inner end of which is provided' with adeeply recessed portion, a'pair of .terminals of different lengthsextending lthrough the end Wall of said lenvelope and through saidinsulator, the inner end of the longer terminal-being exposed beyond theinner end of said insulator and the inner end of the shorter terminalextending substantially the length of said recessed portiony of saidinsulator, a Aplunger of magnetic material responsive to said magneticeld and freely movable in said envelope, a globule of mercury associatedwith said plunger normally in engageme t with the inner end of saidinsulator and lling he recessed portion thereof to bridge saidterminals, said globule of mercury being movable by tlze attractedmovejment of said plunger out of engagementiwith said terminals and acapillary groove in the inside wall of said envelope for delaying the owof,

said mercury globule from said recessed portion upon the attractedmovement of said plunger.

11. In an electromagnetic structure having an electromagnetic field, acontact element comprlsing a sealed envelope, Aan insulator supported inone end of said envelope, the innenend of which sulator, a plunger ofmagnetic material responsive to said magnetic field' and freely 4movablein said envelope, a globule of mercury associated with said plungernormally in engagement vwith said longer terminaland movable .by the atItracted movement of said plunger into the :recessed portion of saidinsulator inlto engagement with said shorter terminal and a capillarygroove in the inside wall of said-envelope for delaying thejow 'of saidmercuryglobule into said recessed portion lof said insulator.

HENRY C. HARRISON. CHARLES E. POLLARD.

